1. Field of the Invention
The present invention relates to an antivibration device.
More particularly, although not exclusively, in its preferred application, the device of the invention is designed for mounting on board of aircraft, in particular in the cabin of a rotary wing aircraft such as a helicopter, but it could naturally be used for any other application where there is a desire to reduce or even eliminate vibration generated by a moving body.
2. Description of the Related Art
The problem of controlling vibration in a helicopter cabin is described in detail in the introduction of French patent No. 2 770 825 (U.S. Pat. No. 6,067,853). In short, the main existing vibration is due to rotation of the main rotor for providing lift and propulsion, and to the air flow along the helicopter fuselage.
In an attempt to absorb this vibration that appears in the cabin, resonator antivibration devices have been proposed that act by resonance at a predetermined frequency in order to reduce the vibration present at said frequency. Nevertheless, their effectiveness is limited since they act only on vibration having the predetermined frequency, and not on vibration at other frequencies.
In addition, devices of that type become ineffective when the vibration that is to be reduced changes frequency.
The antivibration device of the type having a controllable resonator as described in that prior patent enables vibration at varying frequency to be treated. For that purpose, the resonator is used as an actuator in a closed loop system, and vibration measurements from sensors placed on the helicopter are analyzed by a control unit which delivers the control signal to each actuator in such a manner as to minimize the levels of vibration at the measurement points. Although giving advantageous results, that antivibration device presents a certain number of drawbacks such as heavy weight (about 10 kilograms (kg) for each resonator), which is always penalizing in the field of aviation, and high energy consumption if it is to be effective in treating the levels of vibration that are to be encountered in helicopters.
To remedy those drawbacks, antivibration devices of another design have been developed that make use of a rotary balance weight for creating a force that opposes the vibration that is to be reduced. U.S. Pat. No. 5,903,077 and European patent No. 0 337 040 teach such devices.
The antivibration device or modular vibration force generator described in that US patent comprises modules or assemblies each having a motor and gearing driving a pair of co-operating toothed rotors having respective eccentric masses such that the center of gravity of each rotor is not situated on its axis of rotation. Thus, the rotation of each rotor produces a rotary unbalance and, since the rotors are both disposed in the same plane, they produce unbalance that is identical but turning in opposite directions. This results in a sinusoidal force in the direction that is perpendicular to the plane containing the axes of rotation of the rotors, since the components of the unbalances within this plane are opposite and therefore cancel.
In general, the generator comprises two identical modules that are associated to obtain an adjustable resultant force of amplitude that depends on the phase difference between the modules and of frequency equal to the speed of rotation of the rotors.
A command unit controls and regulates the phase difference between the modules and the speed of rotation of the rotors so that vibration of any frequency can be reduced.
The antivibration device described in the above-mentioned European patent comprises in analogous manner two identical assemblies, each comprising two rotors with respective eccentric flyweights, the two assemblies being disposed symmetrically about an axis of symmetry, and said eccentric flyweight rotors having axes that are parallel to one another and orthogonal to the axis of symmetry are rotated by respective motors that are servo-controlled to rotate at the same speed. Such a device can thus easily adapt to fluctuations in amplitude, in phase, and in frequency of the vibration that is to be attenuated.
Although those antivibration devices having rotary unbalance weights provide considerable progress in treating vibration, they nevertheless present drawbacks, and in particular:                an increase in the level of vibration (instead of a decrease) in the event of a breakdown. Should the motors of the two modules cease to rotate at the same speed, for any reason whatsoever, the resulting vibratory force is no longer under control, thereby leading immediately to greatly degraded comfort, which becomes significantly worse than it would be in the absence of any treatment;        a control unit that is quite complex since it needs to synchronize the modules while the motors are getting up to speed, thereby complicating the control algorithms and correspondingly increasing the risk of a breakdown; and        relatively high cost because of the large number of expensive components: several motors, sets of gearing, and a control unit that is complex since it is required to synchronize the speeds of the motors and servo-control their phase difference in order to implement an active system.        